1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly to an optical amplifier that contains a front-end optical amplifier and a back-end optical amplifier, and that is equipped with an automatic compensation function for automatically detecting and compensating for the loss caused by a dispersion compensator inserted between the front-end and back-end optical amplifiers.
2. Description of the Related Art
In recent years, development effort has been exerted to increase the transmission capacities and transmission distances of optical transmission systems using optical amplifiers, and systems with increased capacities and transmission distances are being deployed in practical applications. An optical amplifier using an erbium-doped fiber capable of amplifying light in the 1.55 .mu.m range is the predominant type in use today, and an optical signal in the 1.55 .mu.m range is used as signal light.
Most of the currently installed optical fibers are either 1.3 .mu.m zero dispersion fiber (SMF: Single Mode Fiber) whose zero dispersion wavelength is in the 1.3 .mu.m range or 1.55 .mu.m zero dispersion fiber (DSF: Dispersion Shifted Fiber) whose dispersion characteristic is shifted so as to achieve zero dispersion in the 1.55 .mu.m range. In the U.S. and Europe, 1.3 .mu.m zero dispersion fibers have by far the largest installed base.
FIG. 1 shows dispersion characteristic examples of optical fibers.
Two dispersion characteristic examples, one for 1.3 .mu.m zero dispersion fiber and the other for 1.5 .mu.m zero dispersion fiber, are illustrated in FIG. 1. As shown by dashed lines in FIG. 1, the dispersion characteristic of the 1.3 .mu.m zero dispersion fiber for light at 1.55 .mu.m is about 17 ps/km.nm. This value indicates that when light in the 1.55 .mu.m range is propagated through the 1.3 .mu.m zero dispersion fiber over a distance of 1 km, a propagation time difference of 17 ps will occur at the output end for every nanometer (=0.001 .mu.m) displacement in wavelength.
In an optical transmission system, since transmission signal light generally has a band width of about 0.2 nm or less, if a 1.55 .mu.m optical signal having a band width of 0.1 nm, for example, was transmitted through the 1.3 .mu.m zero dispersion fiber over a distance of 100 km, a propagation time difference or temporal signal light fluctuation of 170 ps would occur at the output end. As an example, if a 10-Gb/s signal was used as the transmission signal, since one cycle of the signal is equal to 100 ps, it is clear that reception would be rendered impossible in the event of the temporal fluctuation of 170 ps. If a lower speed transmission signal was used, on the other hand, the temporal fluctuation would degrade the transmission characteristic.
Accordingly, if signal light at 1.55 .mu.m was transmitted through the currently most widely installed 1.3 .mu.m zero dispersion fiber, dispersion would occur within the optical fiber, and the transmission characteristic would degrade due to the effects of the dispersion. In view of this, when installing an optical amplifier in a 1.3 .mu.m zero dispersion fiber segment, it is practiced to insert in the segment a dispersion compensator capable of compensating for the dispersion characteristic, mostly a dispersion compensating fiber (DCF) having an inverse dispersion characteristic to that of the 1.3 .mu.m zero dispersion fiber.
FIG. 2 is a diagram showing one example of an optical transmission system.
FIG. 3 is a diagram showing the basic configuration of a prior art optical amplifier used in the optical transmission system of FIG. 2.
FIG. 2 shows the case where signal light is transmitted from a transmitting end station 10 to a repeater station 12 through a 1.3 .mu.m zero dispersion fiber (SMF) 11, and then from the repeater station 12 to a receiving end station 14 through a 1.55 .mu.m zero dispersion fiber (DSF) 13. Optical amplifiers 23, 24, and 25 provided at the respective stations 10, 12, and 14 are erbium-doped fiber (EDF) amplifiers, so that signal light of 1.55 .mu.m wavelength is used as the transmission signal light.
As shown in FIG. 3, the optical amplifiers 23, 24, and 25 are each constructed from a two-stage amplifier consisting of a front-end and a back-end amplifier, and a dispersion compensating fiber (DCF) is inserted as necessary between the front-end optical amplifier 31 and the back-end optical amplifier 32. As an example, in the case of FIG. 2, a dispersion compensating fiber is inserted in the optical amplifier 23 at the transmitting end station 10 to compensate for the dispersion that occurs in the 1.3 .mu.m zero dispersion fiber (SMF) 11.
In the other optical amplifiers 24 and 25, the front-end optical amplifier is connected directly (through-connection) to the back-end optical amplifier. This is because the phenomenon of dispersion does not occur when the signal light of 1.55 .mu.m wavelength is transmitted through the 1.55 .mu.m zero dispersion fiber (DSF) 13. The front-end optical amplifier 31 and the back-end optical amplifier 32 are each constructed from an erbium-doped fiber amplifier, and optical output automatic level control (ALC) is performed for each amplifier independently of the other.
However, since the dispersion compensator has in itself an insertion loss of 3 to 10 dB, the optical amplifier gain that becomes necessary for the 1.55 .mu.m zero dispersion fiber segment (where no dispersion compensator is installed) is different from that for the 1.3 .mu.m zero dispersion fiber segment (where the dispersion compensator is installed). In the prior art, therefore, different kinds of optical amplifiers have been used that have different characteristics required for the respective fiber segments. This has lead to the problem of increased amplifier costs and increased expenditures for maintenance and management because of the need to install an optical amplifier that matches each different optical fiber segment.
Since there is a limit on the power that may be input to the dispersion compensating fiber connected to the front-end optical amplifier 31, the optical output power of the front-end optical amplifier 31 is controlled to within the upper limit of the above input power. One factor that limits the input power is stimulated Brillouin scattering, which imposes a limit on the input to the optical fiber. Stimulated Brillouin scattering is a nonlinear effect caused in the output power by stimulated emission when a power exceeding a threshold is coupled into an optical fiber; as a consequence, the signal waveform is distorted, degrading the transmission characteristic.
As a result, in the prior art, the optical output power has been set low also for the 1.55 .mu.m zero dispersion fiber segment where a dispersion compensating fiber need not be connected; this has lead to the problem that the noise figure of the optical amplifier used in the 1.55 .mu.m zero dispersion fiber segment is degraded.
An approximation equation for expressing the noise figure is given as EQU Noise figure=Pase /h.nu..DELTA..nu.G
where Pase: ASE power at signal wavelength
h: Planck's constant PA1 .nu.: Frequency of signal light PA1 .DELTA..nu.: Bandwidth PA1 G: Gain
As can be seen from the equation, the noise figure improves as the gain (optical output) increases.